From MozillaZine Knowledge Base

Mozilla Bookmark Sorter is a graphical Java program which alphabetically sorts Firefox/Mozilla/Netscape formatted bookmark files (usually called bookmarks.html). Here are the installation and usage instructions.

Another option is to use Sort Bookmarks, a Firefox-only extension.

For recent versions of Firefox:

1. Bring up the Bookmarks Manager (Bookmarks -> Manage Bookmarks);
2. Highlight any file or folder in the folder you want to alphabetize;
3. Right-click the highlighted folder and select "Sort by Name".

This procedure only alphabetizes the files in the one folder.